A survey of Fe-isotope fractionation in the marine biogeochemical cycle
Final Report Abstract
We developed preparation and analytical methods to study Fe isotopes within the biogeochemical cycle of Fe in the Gotland Deep, Baltic Sea. The Gotland Deep is an anoxic Basin in the central Baltic Sea, which is ventilated only episodically on a decadal time scale. The focus of this study was therefore on Fe redox processes. The project raised analytical as well as scientific objectives. The scientific objectives comprised three aspects of the marine redox cycle of Fe: a) water column processes, b) estuarine processes and c) sedimentary diagenetic processes, of which two (part a and c) were pursued within the two years funded (out of three requested). We developed a sequential leaching method for reactive Fe phases in sediments and tested them for reliability (precision and accuracy). The leaching procedure consists of a concentrated BaCl2 (or MgCl2) leaching step to retrieve mobile and adsorbed Fe, a dithionite-citrate leaching step to retrieve all reducible reactive Fe(III), and a sequential buffered H2O2-oxidation and citratedithionate reduction step for reactive Fe(II). The method was developed on sediment material from acid mining piles, where all three phases were known to coexist in high abundance. The first two steps can easily be performed in inert atmosphere inside a glove bag, if required. For measurement of seawater, soluble and suspended matter Fe are separated by a two-stage (5 µm and 0.45 µm) filtration. Sample size is ~ 500 ml for the low concentration (~10 nM) surface waters. Much less is required for the Fe-rich anoxic water. Ultimately, sample size will be determined by the suspended matter concentration. Suspended matter Fe is leached from PTFE filters by aqua regia and measured for Fe isotopes with a standard bracketing protocol. Soluble Fe is coprecipitated with Mg(OH)2 from seawater after double-spiking with 54Fe and 58Fe. Here, mass bias correction is done by internal correction, after careful initial blank measurement. The results presented here for the first time (as to our knowledge) show a complete marine profile from oxic to fully anoxic (euxinic). Across the Baltic Sea oxic-anoxic boundary, the water column data show light isotope enrichment in the particulate phase during oxidation – precipitation and heavy isotope enrichment in the residual soluble Fe. This is the opposite of the overall isotope fractionation along the Fe oxidation – precipitation reaction sequence described in previous experimental or natural environment studies so far. The fundamental difference between this study and the others is the marine alkaline pH environment of the Baltic Sea (pH > 7). Previous studies were either observed in naturally or experimentally conditioned acidic environment (pH < 7), whereas the Baltic Sea profile has typical marine pH between 7.4 and 8.2. Previous studies have shown that in alkaline solution, the reaction components are Fe(II)-carbonate complexes rather than Fe2+aq or FeCl2 and that the reaction kinetics are significantly faster. Back-reduction is inhibited by the presence of Fe-oxyhydroxide particles. This results in a different overall fractionation factor that is no longer controlled by rapid isotopic equilibrium between Fe2+(aq) and Fe3+(aq). Inside the anoxic, deeper part of the basin, Fe isotope fractionation appears to be influenced by dissimilatory Fe reduction in the water column. The results suggest that the Baltic Sea Fe supply may be significantly affected by entrainment from the anoxic deep water during winter mixing. The results also have some implications for the interpretation of an Early Proterozoic low ?56Fe spike, which we relate to a change in the ocean's pH from acidic to alkaline as a consequence of proton consumption due to enhanced photosynthesis. In addition, there is some indication from plankton growth experiments for a significant Fe isotope fractionation during Fe uptake by phytoplankton, but the failed analysis of the filtered particles renders this finding somewhat inconclusive. Due to the circumstances described in the report, the study of reactive Fe in Baltic Sea sediments is still in progress. Final results are expected over the course of the next few months.
Publications
- (2006). “Iron Isotopes in Marine Particles From the Baltic Sea – a Profile From the Anoxic Bottom to the Sea Surface.” Eos Trans. AGU, 87(52), Fall Meet. Suppl., Abstract PP21C-1714
Staubwasser, M., Schoenberg, R. et al.
- (2008). “Distribution of iron isotopes in dissolved and particulate iron from the anoxic Gotland Basin in the Baltic Sea.” Eos Trans. AGU, 89(52), Fall Meet. Suppl., Abstract OS23D-1286
Staubwasser, M., Schoenberg, R. et al.
